The expulsion of NOx by an internal combustion engine is typically controlled by limits established by local, state and federal governmental regulations. The formation of NOx constituents must therefore be maintained at least below some threshold limit or level. It is generally understood that the presence of NOx in the exhaust of internal combustion engines is determined by combustion temperature and pressure as well as by the air/fuel ratio (lambda). An increase in combustion temperature causes an increase in the amount of NOx present in the engine exhaust. Therefore, it is desirable to control the combustion temperature in order to limit the amount of NOx present in the exhaust.
One method for limiting or controlling the combustion temperature has been to recirculate a portion of the exhaust gas back to the engine air intake. It is understood that because the exhaust gas has relatively low oxygen content, this results in a combustion mixture that will burn at a lower temperature. The lower combustion temperature, in turn, reduces the amounts of NOx produced during combustion.
It is also desirable to maximize combustion efficiency, which has traditionally been accomplished by running the combustion engine at or near a selected ignition timing. However, it has been noted that unacceptably high levels of NOx typically are produced when the engine operates at or near such conditions. In order to inhibit the formation and emission of NOx, it is necessary to limit the peak combustion pressure to a threshold value.
One known technique for limiting combustion pressure involves the recirculation of exhaust gases through the induction passage of the combustion chamber as an increase in recirculation of exhaust gases will reduce peak combustion pressure, and therefore, the levels of NOx. Accordingly, the formation of undesirable oxides of nitrogen may be reduced by recirculating a portion of the exhaust gas back to the engine air/fuel intake passage so as to dilute the incoming air/fuel mixture with inert H2O, and CO2. The molar specific heat of these gases, and especially of CO2, absorbs substantial thermal energy so as to lower peak cycle temperatures and/or pressures to levels conducive to reducing NOx formation.
While NOx formation decreases as the exhaust gas recirculation (EGR) flow increases such that it represents a threshold percentage of the exhaust gas constituents, this is accompanied by deterioration in engine performance, such as, an increase in engine roughness with increasing EGR. Therefore, one factor limiting the use of EGR is the amount of EGR-induced performance deterioration that can be tolerated before vehicle performance becomes unacceptable.
It has also been known to provide a recirculating control system that utilizes a sleeve having an outlet disposed in an air conduit, where the outlet of the sleeve is positionable along the air conduit to at least partly occlude the exhaust gas inlet and is movable along a portion of the air conduit to vary the extent of occlusion of the exhaust gas inlet in order to regulate flow of exhaust gas into the air conduit. It has further been contemplated that a cross-sectional area of the outlet end of the sleeve may advantageously be reduced and may or may not be positionable to fully occlude the exhaust gas inlet in order to prevent flow of exhaust gas into the air conduit. In addition, a streamlined body may advantageously be disposed in the air conduit. In this manner, throttling of the inlet air flowing through the sleeve occurs in the reduced portion resulting in a venturi.
Use of EGR however, presents additional challenges. For example, on a diesel engine, during some specific circumstances, the exhaust gas pressure ahead of the turbocharger's turbine at times may be greater than the fresh air pressure in the inlet pipe. This can occur during certain engine operating modes such as: 1) high engine speed/high exhaust mass flow. The turbine becomes choked resulting in low turbocharger efficiency and high exhaust pressures upstream of the turbine (higher than boost pressure); and 2) during engine transients/acceleration when exhaust pulses have high amplitude but the turbocharger's inertia has caused the turbocharger not to speed/spool up yet (turbo lag).
During such operating modes, EGR flow would be substantial (exceeding the desired) even without venturi suction effect in the inlet pipe. Accordingly, EGR flow must be limited. An additional EGR-throttling is necessary. During transients, there is also a need to completely shut-off the EGR supply (i.e. to limit smoke).
What is desired therefore, is a control scheme for use with an EGR that allows for increased control of the EGR system under various operating conditions.